1. Field of the Invention
The present invention relates to drilling fluid telemetry systems and, more particularly, to a telemetry system incorporating an oscillating shear valve for modulating the pressure of a drilling fluid circulating in a drill string within a well bore.
2. Description of the Related Art
Drilling fluid telemetry systems, generally referred to as mud pulse systems, are particularly adapted for telemetry of information from the bottom of a borehole to the surface of the earth during oil well drilling operations. The information telemetered often includes, but is not limited to, parameters of pressure, temperature, direction and deviation of the well bore. Other parameter include logging data such as resistivity of the various layers, sonic density, porosity, induction, self potential and pressure gradients. This information is critical to efficiency in the drilling operation.
Mud pulse valves must operate under extremely high static downhole pressures, high temperatures, high flow rates and various erosive flow types. At these conditions, the valve must be able to create pressure pulses of around 100-300 psi.
Different types of valve systems are used to generate downhole pressure pulses. Valves that open and close a bypass from the inside of the drill string to the wellbore annulus create negative pressure pulses, for example see U.S. Pat. No. 4,953,595. Valves that use a controlled restriction placed in the circulating mud stream are commonly referred to as positive pulse systems, for example see U.S. Pat. No. 3,958,217.
The oil drilling industries need is to effectively increase mud pulse data transmission rates to accommodate the ever increasing amount of measured downhole data. The major disadvantage of available mud pulse valves is the low data transmission rate. Increasing the data rate with available valve types leads to unacceptably large power consumption, unacceptable pulse distortion, or may be physically impractical due to erosion, washing, and abrasive wear. Because of their low activation speed, nearly all existing mud pulse valves are only capable of generating discrete pulses. To effectively use carrier waves to send frequency shift (FSK) or phase shift (PSK) coded signals to the surface, the actuation speed must be increased and fully controlled.
Another example for a negative pulsing valve is illustrated in U.S. Pat. No. 4,351,037. This technology includes a downhole valve for venting a portion of the circulating fluid from the interior of the drill string to the annular space between the pipe string and the borehole wall. Drilling fluids are circulated down the inside of the drill string, out through the drill bit and up the annular space to surface. By momentarily venting a portion of the fluid flow out a lateral port, an instantaneous pressure drop is produced and is detectable at the surface to provide an indication of the downhole venting. A downhole instrument is arranged to generate a signal or mechanical action upon the occurrence of a downhole detected event to produce the above described venting. The downhole valve disclosed is defined in part by a valve seat having an inlet and outlet and a valve stem movable to and away from the inlet end of the valve seat in a linear path with the drill string.
All negative pulsing valves need a certain high differential pressure below the valve to create sufficient pressure drop when the valve is open. Because of this high differential pressure, negative pulse valves are more prone to washing. In general, it is not desirable to bypass flow above the bit into the annulus. Therefore it must be ensured, that the valve is able to completely close the bypass. With each actuation, the valve hits against the valve seat. Because of this impact, negative pulsing valves are more prone to mechanical and abrasive wear than positive pulsing valves.
Positive pulsing valves might, but do not need to, fully close the flow path for operation. Positive poppet type valves are less prone to wear out the valve seat. The main forces acting on positive poppet valves are hydraulic forces, because the valves open or close axially against the flow stream. To reduce the actuation power some poppet valves are hydraulically powered as shown in U.S. Pat. No. 3,958,217. Hereby the main valve is indirectly operated by a pilot valve. The low power consumption pilot valve closes a flow restriction, which activates the main valve to create the pressure drop. The power consumption of this kind of valve is very small. The disadvantage of this valve is the passive operated main valve. With high actuation rates the passive main valve is not able to follow the active operated pilot valve. The pulse signal generated is highly distorted and hardly detectable at the surface.
Rotating disc valves open and close flow channels perpendicular to the flow stream. Hydraulic forces acting against the valve are smaller than for poppet type valves. With increasing actuation speed, dynamic forces of inertia are the main power consuming forces. U.S. Pat. No. 3,764,968 describes a rotating valve for the purpose to transmit frequency shift key (FSK) or phase shift key (PSK) coded signals. The valve uses a rotating disc and a non-rotating stator with a number of corresponding slots. The rotor is continuously driven by an electrical motor. Depending on the motor speed, a certain frequency of pressure pulses are created in the flow as the rotor intermittently interrupts the fluid flow. Motor speed changes are required to change the pressure pulse frequency to allow FSK or PSK type signals. There are several pulses per rotor revolution, corresponding to the number of slots in the rotor and stator. To change the phase or frequency requires the rotor to increase or decrease in speed. This may take a rotor revolution to overcome the rotational inertia and to achieve the new phase or frequency, thereby requiring several pulse cycles to make the transition. Amplitude coding of the signal is inherently not possible with this kind of continuously rotating device. In order to change the frequency or phase, large moments of inertia, associated with the motor, must be overcome, requiring a substantial amount of power. When continuously rotated at a certain speed, a turbine might be used or a gear might be included to reduce power consumption of the system. On the other hand, both options dramatically increase the inertia and power consumption of the system when changing from one to another speed for signal coding. Another advantage of the oscillating shear valve is the option to use more sophisticated coding schemes than just binary coding. With the fast switching speed and large bandwidth of the oscillating shear valve, multivalent codes are possible (e.g. three different conditions to encode the signal). The large bandwidth also enables the operator to use chirps and sweeps to encode signals.
The aforesaid examples illustrate some of the critical considerations that exist in the application of a fast acting valve for generating a pressure pulse. Other considerations in the use of these systems for borehole operations involve the extreme impact forces, dynamic (vibrational) energies, existing in a moving drill string. The result is excessive wear, fatigue, and failure in operating parts of the system. The particular difficulties encountered in a drill string environment, including the requirement for a long lasting system to prevent premature malfunction and replacement of parts, require a robust and reliable valve system.
There are also requirements to communicate from the surface to the downhole systems. Such communication may be used to change downhole adjustable drilling parameters, change logging parameters, and to change or adjust the communication parameters between the surface and downhole systems to improve the transfer of data.
Prior art techniques for surface to downhole communication commonly include (i) changing the rotary speed of the drill string, and/or (iii) bypassing a portion of the high pressure output of the drill fluid pumps at the surface. Both techniques have disadvantages. For example, rotary speed changes of the drill string are commonly detected by rotational sensors, such as accelerometers, downhole. Long drill strings, however, are flexible and may exhibit axial, bending, and torsional responses to the forces imposed on the drill string. Changing the rotational speed to send a downlink signal may excite some, or all, of the aforementioned responses. These responses may cause damage to the drill string. In addition, the communication speed is very slow because the large rotational inertia of the entire drill string must be accelerated and decelerated to send a signal. The downhole drill string components are also exposed to rotational speed changes due to the load changes and the stick/slip action of the drilling process. The rotational speed changes caused by the drilling process can easily interfere with the surface generated rotational speed change leading to missed or misinterpreted signals.
Bypassing a portion of drilling fluid from the pump discharge line that supplies the drill string causes a flow decrease and pressure decrease downhole. A downhole flow and/or pressure sensor is commonly used to detect such changes. Typically, the bypass is accomplished with a bypass valve that diverts a portion of the high pressure (on the order of 210 bar/3000 psi) fluid from the supply line back to the suction pit that is typically at atmospheric pressure. This venting action generates very high fluid velocities through the valve, leading to extreme valve erosion. Even wear resistant coatings such as diamond and tungsten carbide show high wear rates causing frequent replacement at relatively high cost. Valve failure due to such erosion can be a safety hazard due to the high pressures involved. The trend to deeper wells leads to even higher supply line pressures (on the order of 490 bar/7000 psi) exacerbating the wear situation. In addition, such bypass systems act at low actuation speeds leading to single pulse signal lengths longer than several seconds. Transmitting data words leads to undesirably long rig delays.
The low actuation speeds of the exemplary prior art downlink systems and methods limit the operator to the use of discrete pulses to send signals. Detection of these pulses is more prone to fail than would be detection of essentially continuous wave form signals using more sophisticated encoding techniques.
The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing a mud pulse telemetry system incorporating a novel downlink system without the need of a bypass valve.